The present invention relates to multiphase brushless DC motors using two Hall-effect generators.
In a prior art four-phase brushless DC motor using two Hall-effect generators electrically mutually displaced by 90.degree., the four stator windings are supplied with currents provided by a respective one of four power transistors each being controlled in response to the switching of a respective one of a plurality of switching transistors. The switching transistors have their emitter electrodes connected in common to form a single differential amplifier and their base electrodes connected to the voltage electrodes of the Hall-effect generators so that the switching time of each switching transistor is determined by the relative Hall voltages of the Hall-effect generators. However, the Hall voltage generated in response to the variation of magnetic fields contains DC voltage components of amplitudes differing between Hall-effect generators because of their nonuniform operating characteristics. Furthermore, the DC voltage components tend to vary as a function of time and other influencing factors so that the result is a variation of the relative amplitudes of the potential at the base electrodes of the switching transistors. As a result, the switching time and therefore commutation internal of each winding may vary as a function of the DC component and hence the operating performance of the Hall-effect generators. This problem will be severe when the waveform of the Hall voltages is a pulse-like shape rather than sinusoidal.